As the use of high strength steels increases in automotive applications, there is a growing demand for steels of increased strength without sacrificing formability. Growing demands for weight saving and safety requirement motivate intensive elaborations of new concepts of automotive steels that can achieve higher ductility simultaneously with higher strength in comparison with the existing Advanced High Strength Steels (AHSS).
Thus, several families of steels like the ones mentioned below offering various strength levels have been proposed.
Among those concepts, steels with micro-alloying elements whose hardening is obtained simultaneously by precipitation and by refinement of the grain size have been developed. The development of such High Strength Low Alloyed (HSLA) steels has been followed by those of higher strength called Advanced High Strength Steels which keep good levels of strength together with good cold formability. However, the tensile levels reached by these grades are generally low.
So as to answer to the demand of steels with high resistance and at the same time high formability, a lot of developments took place. However, it is well known that for high strength steels, trying to increase the ultimate tensile strength generally leads to lower ductility levels. Nevertheless, carmakers keep developing more and more complex parts that require more ductility without sacrificing the resistance requirements. In addition, an improvement in yield strength and hole expansion performance over steels currently in production is needed, for instance for hot dip coated steel sheets.
The US application US2013008570 is known, such application deals with an ultra high strength steel plate with at least 1100 MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the aforementioned soft phase is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1.0. The ultra high strength steel plate has at least 1100 MPa of tensile strength. Such application is silent as regards to different formability issues such as hole expansion and yield strength which have important impact on in use properties.
It is also known the application WO2012153016, dealing with a cold rolled steel which tensile strength is above 1000 MPa and uniform elongation above 12%, as well as V bendability above 90°. The chemical composition of this application comprises, in weight percent: 0.15%≤C≤0.25%, 1.8%≤Mn≤3.0%, 1.2%≤Si≤2%, 0%≤Al≤0.10%, 0%≤Cr≤0.50%, 0%≤Cu≤1%, 0%≤Ni≤1%, 0%≤S≤0.005%, 0%≤P≤0.020%, Nb≤015%, Ti≤0.020%, V≤015%, Co≤1%, N≤0.008%, B≤001% while Mn+Ni+Cu≤3%, the remainder being Fe and inevitable impurities from the cast. The steel microstructure contains, in surface percentage, 5 to 20% of polygonal ferrite, between 10 and 15% of residual austenite, from 5 to 15% of martensite, balance being lath type bainite. This application requires austenite to be stabilized through the continuous annealing process.